1. Field of the Invention
The present invention relates to an electron gun for a cathode ray tube display (CRT), and in particular, to an electron gun for a CRT which enhances the horizontal uniformity of electron beams over the entire screen area by additionally providing a subsidiary electrode and forming a pre-focus lens differentiated in intensity per the respective locations on the screen.
2. Description of Related Art
Generally, a CRT includes an electron gun for emitting electron beams, a deflection yoke for deflecting the electron beams, a shadow mask for color-selecting the electron beams, and a panel with an inner phosphor layer. The electron beams emitted from the electron gun are deflected by the magnetic field of the deflection yoke, and the deflected electron beams pass through the color-selecting shadow mask, and then collide with green, blue and red phosphors to emit light to display the desired images.
The electron gun of the CRT includes a cathode for emitting thermal electrons, a heater installed at the cathode to heat the cathode allowing for the emission of thermal electrons, and a plurality of electrodes for focusing and accelerating the thermal electrons emitted from the cathode. The electrodes include first and second electrodes that form a triode portion with the cathode, a plurality of focusing electrodes receiving focusing voltages, and an anode electrode receiving a high anode voltage.
As the screens of modem CRTs are larger and flatter than before, the center and the periphery of modem CRT screens have a larger variation in image clarity. Particularly, with the widening of the deflection angle (maximally up to 125°) to slim the CRT, the distance between the center and the periphery of the modem CRT screen becomes larger than that of earlier CRT screens having a deflection angle of 102-106°. This can result in poor horizontal uniformity of the electron beams. The horizontal uniformity is deteriorated due to the excessive deflection aberration of the deflection yoke.
In an electron gun, electrons are emitted from a cathode pass a first electrode portion while forming into electron beams. The electron beams are primarily pre-focused at a pre-focus lens portion, followed by passing a dynamic auto-focus lens portion while being pre-diffused. Then, the electron beams pass a main lens portion while being focused, and collide against the phosphor screen of the panel. As the trajectory of the electron beams directed toward the center is different in distance from that of the electron beams directed toward the periphery, the focusing forms (i.e., whether the electron beam is under-focused, over-focused or focusedjust right) ofthe electron beams landing on the screen are different from each other. The electron beams beyond the main lens portion are deflected, and the electron beams are increasingly over-focused at portions on the screen furthest from the center of the screen along the horizontal direction (in the direction of the X axis).
The over-focusing of the electron beams occurs because the electric field lens produced by the deflection yoke of the wide-angled CRT (the influence of the horizontal pin cushion electric field) is strengthened. Such electron beams are focused in the shape of a longitudinal oval with a long horizontal diameter and a short vertical diameter. The electron beams along the horizontal direction of the panel (in the direction of the X axis) are under-focused at the center of the screen, properly focused (i.e., focused just right) at the ½ location ofthe screen, and an over focused at the left and right ends of the screen. As proper focusing occurs at the ½ location (between the center and ends on the X axis) of the screen, the beam diameter is too small compared to the pitch of the shadow mask, and a ring-shaped moire induced by the interference is generated, thus deteriorating the display image quality. Therefore, what is needed is an improved design for an electron gun for a CRT that is better suited for today's large screen flat panel designs that overcomes this moire at the ½ location and reduces over-focusing at the edges of the CRT along the horizontal axis of the CRT.